Air compressor surge control apparatus



Oct. 21, 1969 J. M. EASTMAN 3,473,727

AIR COMPRESSOR SURGE CONTROL APPARATUS Filed Jan. 2, 1968 3 Sheets-Sheet l I A I24 E5 INVENTOR. 56' l 0 BLEEQ VALVE JAMES M- EASTMAN BY zf asq 2 AGENT Oct. 21, 1969 J. M. EASTMAN 3,473,727

AIR COMPRESSOR SURGE CONTROL APPARATUS Filed Jan. 2, 1968 3 Sheets-Sheet 2 7a 4014 29:556. 4/1.? 5002C P4 86 fig -Q 2 k: 156 /54 55 134 "E Q .F'Ig?! 5 INVENTOR 7r 4 J JAMES M EASTMAN AGENT Oct. 21, 1969 J. M. EASTMAN 3,473,727

AIR COMPRESSOR SURGE CONTROL APPARATUS Filed Jan. 2, 1968 3 Sheets-Sheet 5 6' & W 4

l ia 55 '1 a 20 I E l 26/ '4 WWW;

Pt 3 0 I a 2 0/94 20 40 G0 I00 W 44i} 8,1 PER CENT DESIGN INVENTOR.

J JAMES M. EASTMAN United States Patent O 3,473,727 AIR COMPRESSOR SURGE CONTROL APPARATUS James Middleton Eastman, South Bend, Ind, assignor to The Bendix Corporation, a corporation of Delaware Filed Jan. 2, 1968, Ser. No. 695,245 Int. Cl. F04d 27/02; F0413 49/00, 39/00 US. Cl. 230-115 Claims ABSTRACT OF THE DISCLOSURE wherein K represents a predetermined constant related to a given empirical function defining the compressor surge line on a compressor map.

BACKGROUND OF THE INVENTION Compressor air bleed control mechanisms used in avoiding surge or stall of a multi-stage axial flow compressor is well known in the compressor art. Such control mechanism has taken various forms among which are those which provide pressure sensing means responsive to various compressor air pressures and/ or compressor air pressure ratio, air temperature sensing means, compressor rotational speed sensing means which have a predetermined relationship relative to stall or surge conditions likely to develop within one or more of the compressor stages. In general, the sensing of pressure ratios in an air compressor as well as the relating of one or more compressor air pressures, against a sensed pressure ratio, air temperature and/or compressor speed, for control purposes requires a relatively complicated array of compoent elements which tend to render the united air bleed control mechanism bulky, weighty and sometimes unreliable as well as correspondingly expensive to manufacture.

A further disadvantage of prior art air bleed control mechanisms exists by virtue of the relatively wide margin of safety necessitated by the control mechanism in exercising control over the bleeding of air from the compressor under potential surge or stall conditions of the compressor, thereby adversely affecting compressor efficiency. Also, existing air bleed control mechanisms are not entirely satisfactory due to the lack of control adaptability, without widespread modifications thereto, to compressors having difierent design pressure ratios and exhibiting difierent surge or stall characteristics.

SUMMARY OF THE INVENTION The present invention provides air compressor bleed control mechanism which senses proximity to a predetermined compressor operating condition such as compressor surge or stall by means of the functional relationship between compressor inlet to discharge pressure ratio and compressor inlet total to static pressure ratio or compressor inlet to discharge pressure ratio and compressor discharge total to static pressure ratio. The necessity for sensing pressure ratios directly is eliminated by the use of pressure differential responsive means responsive to an air pressure common to both pressure ratios thereby providing a closely approximated schedule of the abovementioned ratio relationship in terms of a proportionality between total to static pressure difference and ice compressor pressure rise. The precision of the approximate schedule relative to matching the compressor surge or stall curve may be implemented by the use of various means such as series fiow air restrictions, boost venturis, difiusers and/ or ratio of areas of the pressure differential responsive means which provide for independent adjustment of two points as well as the concavity of the curve defining the abovementioned approximated schedule. The present invention is adapted to utilize fluidic devices as a substitute for diaphragms or the like pressure responsive members thereby providing further simplified air bleed control mechanism.

It is therefore an object of the present invention to provide a compressor air bleed control mechanism which is simple in structure, reliable and accurate.

It is another object of the present invention to provide control mechanism for a gas turbine engine having an air compressor, which control mechanism senses proximity to a compressor condition to be avoided by means of a functional relationship between a ratio of air pressures representative of air flow and compressor pressure ratio without sensing the pressure ratios directly.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 represents a schematic drawing partly in section of a gas turbine engine and control mechanism therefor embodying the present invention;

FIGURE 2 represents a schematic drawing partly in section of a gas turbine engine and control system therefor embodying a modified form of the present invention;

FIGURE 2a represents a schematic drawing partly in section of a modified form of the invention shown in FIGURE 2; I

FIGURE 3 represents a schematic drawing partly in section of a second modified form of the present invention;

FIGURE 4 represents a schematic drawing partly in section of a third modified form of the present invention;

FIGURE 5 represent a compressor map showing constant corrected speed (N 6 curves and constant compressor discharge corrected air flow with a compressor surge or stall curve plotted in terms of compressor pressure ratio P /P as a function of compressor inlet corrected air flow V m W FIGURE 6 represents a compressor map showing constant corrected speed N/ /0, curves and a compressor surge line plotted in accordance with the relationship of compressor pressure ratio P /P as a function of compressor inlet corrected air flow DESCRIPTION OF THE PREFERRED EMBODIMENTS The following table lists the symbols with corresponding definitions of the various parameters of operation involved in the equations of operation discussed herein.

P =compressor inlet total pressure (absolute) P =compressor inlet static pressure (absolute) T =compressor inlet total temperature (absolute) a zcompressor inlet total pressure divided by standard atmospheric pressure (14.7 p.s.i.a.)

=compressor inlet total temperature divided by standard atmospheric temperature (519 R.)

P =compressor discharge total pressure (absolute) P =compressOr discharge static pressure (absolute) T =compressor discharge total temperature (absolute) d =compressor discharge total pressure divided by standard atmospheric pressure 14.7 p.s.i.a.)

0 =compressor discharge total temperature divided by standard atmospheric temperature (519 R.) =compressor mass air flow N=compressor rotational speed Referring to FIGURE 1, numeral 20 designates a conventional gas turbine engine having an air inlet 22 leading to an axial flow compressor 24 which discharges compressed air to a plurality of combustion chambers 26 adapted to receive pressurized fuel from a conventional fuel control, not shown, which fuel and air when mixed and ignited in the chambers 26 provides a source of hot motive gas which passes through a gas turbine 28 to drive the same and subsequently exhausts through an exhaust nozzle 30 to the atmosphere thereby providing thrust. The turbine 28 is connected to drive compressor 24 via a shaft 32 suitably mounted for rotation.

Referring to FIGURE and the compressor surge characteristic defined by the curve labeled surge line, it will be recognized that the area above the surge line represents an operating region of the compressor wherein the charatceristics undesirable unstable-operating condition of an axial flow compressor commonly known as surge or stall is likely to occur and from which region the compressor must be excluded if performance deterioration as well as permanent structural damage to the compressor and engine is to be avoided. To that end, the present invention in the embodiment of FIGURE 1 includes a casing 34 separated into a plurality of chambers 36, 38 and 40 by spaced apart diaphragms 42 and 44 having radially outermost portions fixedly secured to casing 34 by suitable conventional fastening means, not shown, providing an air tight seal. A valve member 46 is provided with a stem 48 fixedly secured to the center most portion of diaphragms 42 and 44 by any suitable conventional fastening means, not shown, providing an air tight seal. The valve member 46 is actuated by forces derived from diaphragms 42 and 44 in response to air pressures acting thereagainst and cooperates with an orifice 50 to define a variable area flow inlet to chamber 36. An outlet passage 52 communicates chamber 36 with a conventional pitot tube generally indicated by 54 located in the compressor inlet 22 and exposed to he total air pressure P of the incoming air. A passage 60 communicates chamber 40 with a conventional pressure sensor generally indicated by 58 exposed to static air pressure P at the compressor discharge section. A passage 56 communicates chamber 38 with a conventional pitot tube generally indicated by 62 located in the discharge section of compressor 22 and exposed to the total air pressure P of the discharge air.

A compressor discharge air bleed valve is housed in a casing 64 provided with an inlet port 66 communicating with a passage 68 leading to the discharge section of compressor 22 and an outlet port 70 leading to a relatively low air pressure drain source such as the atmosphere at pressure P Communication between inlet port 66 and outlet port 70 is controlled by a normally closed valve member 72 which engages a valve seat 74 and which is fixedly secured to a diaphragm 76 by any suitable conventional fastening means, not shown. A compression spring 78 interposed between diaphragm 76 and casing 64 serves to preload valve member 72 to the closed position shown. An axially located passage 80 in valve member 72 is provided with a restriction 82 and communicates inlet port 66 with 7 a chamber 84 partially defined by diaphragm 76 which chamber 84 is vented to valve member 46 via a passage 86, The position of valve member 46 which determines the effective flow area of orifice 50 in flow controlling relationship with passage 86 controls the air pressure P level in chamber 84. The P pressure acts against diaphragm 76 in opposition to the pressure Ptg acting against the relatively smaller area of valve 72 exposed thereto plus the drain air pressure P acting against the diaphragm 76 area exposed thereto minus the valve 72 area exposed thereto. In the event of pressures P P and P being equal, the spring 78 serves to hold valve 72 in a closed position against seat 74. With the compressor 24 in operation and discharge air at a corresponding relatively high pressure P available, the valve 72 will remain in a closed position until the valve 46 opens sufiiciently to establish the necessary pressure differential P -P across restriction 82 to overcome the spring 78 as will be described hereinafter.

Referring to FIGURE 2, a modified form of the present invention is shown wherein numeral 88 represents a casing having a pair of spaced apart diaphragms 90 and 92 which are fixedly secured thereto at their radially outermost edge portions by suitable conventional fastening means, not shown, providing an air tight seal. The diaphragms 90 and 92 together with casing 88 define a plurality of chambers 94, 96 and 98. Chamber 98 is vented via passage 60 to compressor discharge static P Chamber 96 is vented via a passage 104 to a pitot tube 106 suitably located in the throat of venturi 102 and exposed to total pressure P of the compressor discharge air passing thereto. A passage 108 containing a pair of restrictions 110 and 112 in series flow relationship connects chamber 96 with passage 52 leading to pitot tube 54 at compressor inlet total air pressure P The diaphragms 90 and 92 are fixedly secured by any suitable fastening means, not shown, to a stem 114 which, in turn. is fixedly secured to the movable end of an evacuated bellows 116 having its opposite sealed end anchored to casing 88. A passage 118 vents chamber 94 to passage 108 intermediate restrictions 110 and 112 at air pressure P1 which varies as a predetermined function of air pressures P and Pa upstream from restriction 112 and downstream from restriction 110, respectively, as will be described hereinafter.

A lever 120 it pivotally secured at one end to stem 114 and at its opposite end carries a servo valve member 122 which coacts with a valve orifice 124 to define a variable area outlet for passage 86 connected thereto. The orifice 124 vents passage 86 to a cavity 126 which, in turn, is connected via a passage 128 to passage 52 at compressor inlet total air pressure P The passage 86, as in the case of FIGURE 1 heretofore described, is connected to chamber 84 in casing 64 housing the compressor discharge air bleed valve 72.

Referring to FIGURE 2a, the embodiment of FIGURE 2 is shown modified to the extent that chamber 98 is vented via a passage 100 to static air pressure P' at venturi 102 located to receive air discharged by the compressor 24. The chamber 96 is vented via a passage 104 to a pitot tube 106 suitably located in the throat of venturi 102 and exposed to total pressure P' of the compressor discharge air passing through venturi 102.

Referring to FIGURE 3, a third embodiment of the present invention is shown wherein numeral 130 designates a casing divided interiorly into three chambers 132. 134 and 136 by spaced apart diaphragms 138 and 140 fixedly secured at the radially outermost portions thereof to casing 130 by any suitable conventional fastening means, not shown, providing an air tight seal. Chamber 134 is vented via a passage 142 to compressor inlet static air pressure P Chamber 136 is vented via a passage 144 to pitot tube 54 at compressor inlet total air pressure P A passage 146 provided with restrictions 148 and 150 having a predetermined area ratio communicates pitot tube 62 at compressor discharge total air pressure P with passage 142 at compressor inlet static air pressure P Chamher 132 is vented via a passage 152 to passage 146 intermediate restrictions 148 and 150. The diaphragms 138 and 140 are secured to a stem 154 by any suitable conventional fastening means, not shown, providing an air tight seal. A valve member 156 fixedly secured to the end of stem 154 cooperates with an orifice 158 to vent passage 86 connected to orifice 158 to chamber 136 at compressor inlet total air pressure P As in the case of FIG- URE 1, passage 86 is connected to chamber 84 in casing 64 housing the compressor discharge air bleed valve 72.

Referring to FIGURE 4, a fourth embodiment of the present invention is shown wherein the previously described diaphragms and valve member attached thereto are replaced by pure fluid valve means thereby minimizing the mechanical elements of the system with attendant advantages as will be readily apparent to those skilled in the art. To that end, a conventional pulse operated fluid jet switching valve means is shown having an input port 160 connected to a source of pressurized fluid at regulated constant pressure P which may be derived from compressor discharge air. A fluid jet issued by port 160 passes through a channel 162 having opposing fluid control ports 164 and 166 which are aligned and adapted to inject fluid transversely against the fluid jet passing therebetween to deflect the same toward one or the other of two output passages 168 and 170 depending upon the relative output flow diiferential of control ports 164 and 166. Dowstream from the control ports 164 and 166, the side- Walls of channel 162 are provided with recesses 172 and 174 which function to eliminate the tendency of the fluid jet to lock on one or the other of the channel walls as a result of the well known Coanda effect depending upon the direction of deflection of the fluid jet thereby controlling the deflection of the fluid jet and thus pressurization of the output passages 168 and 170 in proportion to the output differential between control ports 164 and 166.

The control port 164 is connected via a passage 176 to the pitot tube 54 at compressor inlet total air pressure P The recesses 172 and 174 are connected via a passage 178 to the compressor inlet 22 at compressor inlet static air pressure P A passage 180 containing restrictions 182 and 184 connects pitot tube 62 at compressor discharge total air pressure P with passage 178 at compressor inlet stati air pressure P A passage 186 containing a restriction 188 connects control port 166 with passage 180 intermediate restrictions 182 and 184.

The output passages 168 and 170 are connected via passages 190 and 192, respectively, to chambers 194 and 196 in a casing 198. A diaphragm 209 separating chambers 194 and 196 and responsive to the pressure difierential therebetween is connected to a normally closed valve 202 engageable with valve seat 264 which is adapted to vent compressor discharge air to drain source pressure P Operation where f is defined by the compressor surge curve.

FIGURE 5 includes curves of constant compressor discharge corrected air flow defined by V t3/ t3 which is related to the compressor inlet corrected air flow by the relationship.

6 5 HAW/ .2 (Wins/m3) V ts/ L2 UH/W For a nominal 85% compressor efliciency r3/ t2E( ts/ z2) permitting the relationship from which the constant curves of FIGURE 5 were plotted.

Since there exists a unique relation between compressor pressure ratio, P tz, and corrected compressor discharge air flow,

on the compressor surge curve, an alternate control function for avoiding entry into the compressor surge area is defined by wWu/mgfsu e/ w) where f is defined by the compressor surge curve.

is proportional to compressor discharge Mach number so that m t3 f Q3/ 53) Combining the latter two equations results in t3 s3fs3( t3 t2) which relationship being in terms of readily available compressor pressures represents a practical mode of operation for compressor surge control. However, it will be recognized that sensing air pressure ratios directly is diiiicult and requires a relatively complicated array of structure which, for obvious reasons, is undesirable.

It will be noted that P occurs in both ratios of equation (3) so that the form of the function i may be adapted to permit sensing air pressure difierentials in place of ratios. Various known mathematical approaches are available to develop an equation approximating a given empirical function. Thus, the functional relationship of equation (3) may be modified to t3 s3 ts r2) The validity of equation (4) as a functional relation between the ratios P /P and P /P may be determined by dividing each side of the equation by P resulting in ts/ m Now, selecting K for a point corresponding to corrected compressor inlet air flow on the compressor surge curve of FIGURE 1, the corresponding pressure ratio is approximately 2.63. Assuming compressor discharge Mach number at the selected point to be .15, the ratio of compressor discharge to static air pressures, P /P is 1.016. Therefore, from equation or K=.0258. Curve A of FIGURE 5 illustrates equation (4) plotted in terms of K=.0258 wherein a value of P /P is computed for each compressor pressure ratio P /P The compressor discharge Mach number is deter- 7 mined from the computed ratio P /P The percent compressor discharge corrected air flow, Wx/e /fi is one hundred times compressor discharge Mach number divided by the maximum Mach number value of .15. Curves B and C of FIGURE illustrate the effect of higher assumed compressor discharge Mach numbers of .5 and .85, respectively for the 100% corrected air flow condition. It will be noted that the concavity of the curves A, B and C is reduced with increased Mach number.

FIGURE 1 represents a simple pressure differential sensing unit capable of operating in accordance with equation (4) above. The value of K is established by the ratio of areas of diaphragms 42 and 44 which may be selected to give the desired approximation of a given compressor surge curve. In FIGURE 1, the valve 72 is normally held in a closed position against seat 74 by the spring 78 aided by the air pressure P in chamber 84 which acts against diaphragm 76 thereby preventing compressor discharge air from passing through valve 72 to the drain source at relatively low air pressure P The compressor discharge air is vented through restriction 82 to chamber 84 which, in turn, is vented to valve 46 via passage 86. The valve 46 being attached to diaphragms 42 and 44 is positioned in accordance with the relationship where A; and A represent the effective areas of diaphragms 42 and 44, respectively, which are selected to provide the constant K of equation (4). The valve 46 will move in an opening direction when the pressure differential relationship exceeds the selected area ratio A /A thereby venting passage 86 to passage 52 causing a decrease in pressure P in chamber 84 and a corresponding increased P -P pressure drop across restriction 82 whereupon valve 72 opens to vent passage 68 to drain air pressure P A thereby increasing corrected compressor discharge air flow accordingly to maintain compressor operation within the safe operating range defined by the compressor surge curve of FIGURE 5. It will be understood that a decrease in the pressure differential relationship below the selected area ratio loads the valve 46 in a closing direction thereby maintaining the valve 72 in a closed position.

Referring to FIGURE 2, the compressor pressure rise P P is applied across the series restrictions 110 and 112 producing intermediate pressure P' It can be shown (see Patent No. 2,950,596 issued Aug. 30, 1960 to E. A. Haase, et al.) that P' =P f (P /P where the function f depends upon the area ratio of the restrictions 110 and 112. If P -P is substituted for P P in equation (4) the result is which, in dividing both sides by P results in Again, a valid functional relation between Ft3/P 3 and P /P is demonstrated. By suitable selection of restrictions 110 and 112 to provide the proper area ratio thereof, the curvature of a line defined by equation (4) may be modified as desired. For example, selecting restrictions 110 and 112 with an area ratio of 1.5 with 100 percent corrected air flow, compressor discharge Mach number of .5 and K corresponding to 100 percent corrected air flow, the curve D is obtained. The concavity of curve D relative to curves A, B and C will be noted. The curves A, B, C and D converge at P /P, =1 and and the shapes thereof depend upon the value of K. As pointed out heretofore, the concavity of the curves A. B, C and D vary with the sensed compressor discharge Mach number, which concavity may be further increased utilizing the series restrictions 110 and 112 as indicated by curve D.

The curve D may be raised or lowered to permit additional flexibility in matching the compressor surge curve of FIGURE 5. To that end, Equation 4 may be modified to P 5sK (P; K P' and subsequently divided by P to give 1 Pa/Pf 1 ut/ us It will be recognized from Equation 8 that curves defined thereby will converge at P /P, =K and W /0, /6 =l) thereby permitting raising or lowering of the curves by suitable selection of K while the slope remains adjustable with K. Curve E of FIGURE 5 represents a plot of Equation 8 with K equal to 1.1 and K selected from the compressor surge line at corrected air flow and compressor discharge Mach number of 0.5.

As shown in FIGURE 2a, additional flexibility of the curves A, B, C or D and/or more accurate sensing of the pressure differential Pug-P1 2 may be obtained by the use of the venturi 102. The venturi 102 permits sensing a compressor discharge Mach number higher than the actual valve by a fixed ratio thereby reducing the concavity of curve D, for example, accordingly. The venturi 102 may be replaced by a diffuser 190 indicated in dashed outline in FIGURE 2:: to obtain increased concavity of curve D, for example, by sensing a compressor discharge Mach number lower than the actual value. In FIGURE 2, compressor discharge air pressures P and P are communicated to chambers 96 and 98 respectively, where the P P pressure differential is sensed by diaphragm 92. The air at pressure P passes through restrictions and 112 to compressor inlet total pressure P at passage 128 with the pressure P generated intermediate restrictions 110 and 112 varying in the heretofore mentioned manner as a function of the area ratio of the restrictions. The pressure P is vented to chamber 94 where it acts against opposing bellows 116 and diaphragm 90. The ratio of the effective end area of bellows 116 to the area of diaphragm 90 corresponds to the value of K in Equation 8 and may be selected as desired to raise the curve as shown by E of FIGURE 5. It will be noted that the P' P pressure differential acting across diaphragm 92 is opposed by the P' P pressure differential across diaphragm 90 plus the P pressure acting against bellows 116 thereby stabilizing lever 120 and valve 122 attached thereto when the pressure differentials attain a predetermined relationship in accordance with the area ratio of the diaphragms 90 and 92 in a manner similar to that of FIGURE 1 heretofore described. However, the pressure differential P' P' acting across diaphragm 92 is opposed by the pressure differential P P which acts upon diaphragm 90 having an effective area ratio established by bellows 116 which, in turn, causes lever 120 to be loaded and actuate valve 122 in an opening direction at a corresponding predetermined pressure differential. As in the case of FIGURE 1 heretofore described, the pressure P in chamber 84 decreases in response to opening movement of valve 122 causing valve 72 to open thereby venting passage 68 to drain air pressure P The embodiment of FIGURE 2a operates in a manner heretofore set forth with regard to FIGURE 2 with the exception that venturi 102 generates a pressure differential P P which, for a given air flow discharged by compressor 24 is greater than the pressure differential Pt3-P 3 of FIGURE 2, results in a sensed compressor discharge air Mach number higher than the actual valve by a predetermined fixed ratio depending upon the venturi 102 characteristics.

The diffuser 190 of FIGURE 2a operates in a reverse manner in comparison to the venturi 102 in that the lower pressure differential P' P' generated thereby represents a lower sensed air flow Mach number than in the case of FIGURE 2 for a given compressor air output.

Referring to FIGURE 3, the embodiment shown therein is similar in operation to the of FIGURE 1 although compressor inlet static pressure P instead of compressor discharge static pressure P is sensed. The larger diaphragm 140 is responsive to the pressure differential P P and the smaller diaphragm 138 is responsive to the differential between pressure P intermediate series restrictions 148 and 150 and pressure P The relationship defined by Equation 4 may be rewritten as 9 P P =K(P P wherein K is the area ratio of diaphragms 138 and 140 which relationship results in P /P :K-l/[K(P /P )1]. Curve G of FIGURE 6 shows the curve generated by applying the P P pressure diiferential across the larger diaphragm 140 and the differential between pressure P and pressure P' generated intermediate series restrictions 148 and 150 across the smaller diaphragm 138 assuming an area ratio of 1.5 for restrictions 148 and 150 and a 0.5 maximum compressor inlet Mach number. The relative concavity of curves obtained with and without the series restrictions 148 and 150 are shown by the curves G and F, respectively, of FIGURE 6. While compressor discharge Mach number sensing may be advantageous by virtue of higher available total to static pressure difierential, the sensing of compressor inlet Mach number may be preferred for certain applications such as compressors having variable air inlet guide vanes or compressor interstage air bleed otf.

Referring to FIGURE 4, there is shown the arrangement of FIGURE 3 modified to accommodate pure fluid switching means in place of the diaphragms 138 and 140. The value K of Equation 9 is determined by the relative areas of ports 164 and 166 through which air is passed to impinge the fluid jet issuing from port 160 thereby deflecting the fiuid jet toward output passage 168 or 170 depending upon which port 164 or 166 provides the higher pressure control flow. Since the value of P /P at which air flow becomes zero cannot be increased as by the use of the bellows 116 of FIGURE 2, an equivalent curve adjust ment is obtained by the series restrictions 182 and 184 across which the pressures P and P are applied. If the downstream restriction 182 is choked, the pressure ratio P /P across the upstream restriction 184 is constant and the intermediate pressure P' considered as K the value of which depends upon the area ratio of restrictions 182 and 184. Assuming a relatively large air flow through the restrictions 182 and 184, a small quantity of air may be tapped off via passage 186 to port 166 for control purposes without upsetting the abovementioned constant relationship. Applying the relatively small air flow through restriction 188 to port 166 results in a control function equivalent to P P ;K(K P -P providing the restriction 182 is choked. The resulting curve would behave as though its P /P intercept at zero air flow is l/K until the compressed pressure ratio Ptg/Ptz becomes low enough to unchoke restriction 182 whereupon the curve would decrease down to P /P =1.0 at zero flow. The pressure differential at output passages 168 and 170 is applied to diaphragm 200 causing it to open valve 202 to provide the desired bleeding of compressor discharge air to drain air pressure P While the above described operation is in terms of bleeding otf compressor discharge air to avoid compressor surge, it will be recognized that alternate control functions of the above described structure to alleviate compressor surge would be to exercise control over fuel flow to the engine, turbine inlet guide vanes or the like. Also, it will be recognized by those persons skilled in the art that the heretofore described approach for preventing compressor surge may be utilized to regulate the steady state operating curve of a compressor as, for example, on the maximum efliciency line which normally is located below and roughly parallel to the compressor surge curve.

I claim:

1. Control apparatus for a multi-stage air compressor having a characteristic range of unstable operation, said control apparatus comprising:

(a) conduit means connected to said air compressor and a relatively low pressure air source for bleeding compressor pressurized air to said source;

(b) normally closed valve means in said conduit means for controlling said compressor pressurized air flow therethrough;

(c) actuating means operatively connected to said normally closed valve means for actuating the same; and

((1) control means operatively connected to said actuating means for energizing the same including:

spaced apart first and second air presure differential responsive members partially defining first, second and third air chambers;

said first chamber being vented to compressor inlet total air pressure;

said second chamber being vented to compressor outlet total air pressure;

said third chamber being vented to compressor outlet static air pressure;

said first pressure diiferential responsive member being exposed to said first and second chambers and responsive to the air pressure difierential therebetween;

said second pressure dilferential response member being exposed to said second and third chambers and responsive to the pressure differential therebetween and having a relatively larger effective area in comparison to that of said first pressure ditferential responsive member;

said first and second pressure diflerential reponsive means being operative to energize said actuating means and open said normally closed valve means to bleed compressor pressurized air to said source to avoid said characteristic range of unstable operation upon a predetermined variation in said total and static air pressures in accordance with the relationship P P ;K(P P wherein P designates outlet total air pressure of said compressor, P designates outlet static air pressure of said compressor, P designates inlet total air pressure of said compressor and K designates a predetermined constant derived from the area ratio of said first and second pressure differential responsive members.

2. Control apparatus for a multi-stage air compressor having a characteristic range of unstable operation, said control apparatus comprising:

(a) conduit means connected to said air compressor and a relatively low pressure air source for bleeding compressor pressurized air to said source;

(b) normally closed valve means in said conduit means for controlling said compressor pressurized air flow therethrough;

(c) actuating means operatively connected to said normally closed valve means for actuating the same; and

(d) control means operatively connected to said actuating means for energizing the same including:

a casing;

first and second air pressure responsive members connected in force opposing relationship and divdding said casing into first, second and third air chambers;

an evacuated bellows in said first chamber and anchored at one end with an opposite movable end connected to said first and second pressure responsive members; passage means connecting said third chamber with compressor outlet static air presure;

conduct means including first and second flow restrictions in series flow relationship communicating outlet total air pressure of said compressor with inlet total air pressure of said compressor;

said first chamber being vented to said last named conduit means; intermediate said first and second flow restrictions;

said second chamber being vented to said last named conduit means upstream from said first flow restrictions;

said third chamber being vented to outlet static air pressure of said compressor; and

means operatively connecting said first and second pressure responsive members and said bellows with said actuating means to energize the latter;

said first pressure responsive member separating said second and third chambers and being responsive to the differential between said compressor outlet total and static air pressures therein, respectively;

said second pressure responsive member separating said first and second chambers and being responsive to the differential between compressor outlet total air pressure and the air pressure intermediate said first and second flow restrictions therein, respectively, and act ing in opposition to said first pressure responsive member;

said first and second pressure differential responsive members and said bellows being operative to energize said actuating means and open said normally closed valve means to bleed compressor pressurized air to said source to avoid said characteristic range of unstable operation upon a predetermined variation in said compressor total and static air pressures.

3. Control apparatus as claimed in claim 2 and further including:

a venturi connected to receive compressor outlet air flow;

said last named conduit means and said third chamber being connected to said venturi to receive venturi velocity and static air pressure, respectively.

4. Control apparatus as claimed in claim 2 and further including:

a difluser connected to receive compressor outlet air flow;

said last named conduit means and said third chamber being connected to said difluser to receive diffuser velocity and static air pressures, respectively.

5. Control apparatus for a multi-stage air compressor having a characteristic range of unstable operation, said control apparatus comprising:

(a) conduit means connected to said air compressor and a relatively low pressure air source for bleeding compressor pressurized air to said source;

(b) normally closed valved means in said conduit means for controlling said compressor pressurized air flow therethrough;

(c) actuating means operatively connected to said normally cosed valve means for actuating the same; and

(d) control means operatively connected to said actuating means for energizing the same including:

a casing;

first and second air pressure responsive members connected in force opposing relationship and driving said easing into first, second and third chambers;

first passage means connecting said third chamber with compressor inlet total air pressure;

second passage means connecting said second chamber with compressor inlet static air pressure;

third passage means connecting said second passage means with compressor outlet total air pressure;

first and second flow restrictions having a predetermined area ratio and connected in series flow relationship in said third passage means;

fourth passage means connecting said first chamber with said third passage means intermediate said first and second restrictions; and

means operatively connecting said first and second pressure responsive members with said actuating means to energize the latter;

said first pressure responsive member separating said third and second chambers and being responsive to the differential between said compressor inlet total and static air pressures therein, respectively;

said second pressure responsive member separating said second and first chambers and being responsive to the diiferential between said compressor inlet static air pressure and the air pressure intermediate said first and second flow restrictions, respectively;

said first and second pressure responsive members being operative to energize said actuating means and open said normally closed valve means to bleed compressor pressurized air to said source to avoid said characteristic range of unstable operation upon a predetermined variation in said compressor total and static air pressures.

6. Control apparatus for a multi-stage air compressor having a characteristic range of unstable operation, said control apparatus comprising:

(a) conduit means connected to said air compressor and a relatively low pressure air source for bleeding compressor pressurized air to said source (b) normally closed valve means in said conduit means for controlling said compressor pressurized air flow therethrough;

(c) actuating means operatively connected to said normally closed valve means for actuating the same; and

(d) control means operatively connected to said actuating means for energizing the same including:

pure fluid means connected to a source of relatively high pressure air and adapted to generate a power air jet;

first and second control ports on opposite sides of said power air jet and arranged to deflect the same in proportion to the control air jet pressure diiferential generated transversely thereto;

first and second output passages adapted to receive said power jet to a variable degree dependent upon the degree of deflection of said power jet relative thereto and the predominating control air jet pressure of the control air jet pressure diflerential;

first passage means connecting said first control port to compressor inlet total air pressure;

third and fourth ports having fluid communication with said power air jet and disposed on opposite sides of said power jet intermediate said first control port and said first output passage and said second control port and said second output passage, respectively;

second passage means connecting said third and fourth ports to compressor inlet static air pressure;

third passage means connecting said second passage means with compressor outlet total air pressure;

first and second restrictions having a predetermined flow area ratio connected in series fiow relationship in said third passage means;

fourth passage means connecting said second control port to said third passage means intermediate said first and second restrictions;

said actuating means including air pressure responsive means operatively connected to said first and second output passages and responsive to the air pressure differential therebetween;

said actuating means being operative to open said normally closed valve means to bleed compressor pressurized air to said source to avoid said characteristic 13 range of unstable operation upon a predetermined variation in said compressor total and static air pressures.

References Cited UNITED STATES PATENTS 2,813,672 11/1957 Long et a1. 230-115 X 2,886,968 5/1959 Johnson et a1. 230114 14 3,047,210 7/1962 Best 2301 14 3,080,712 3/1963 Wood 230-115 3,248,043 4/1966 Taplin 230-115 WILLIAM L. FREEH, Primary Examiner US. Cl. X.R. 230-22, 114

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,473,727 October 21, 1969 James Middleton Eastman It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 49, "he" should read the Column 4, line 23, after "static" insert pressure lines 24 to 27, "Chamber 96 is vented via a passage 104 to a pitot tube 106 total pressure P' of the compressor discharge air passing thereto." should read Chamber 96 is vented via passage 56 to compressor discharge total pressure P Column 8, line 18, "P both occurrence, should read P' line 34, "valve': should read value line 45, "P' should read P' line 48, "P' should read P" line 55,

u I 1 n 1 n n I n P t3 P t2 should read P t3 P t3 line 56, P t2 should read P" line 63, "P' P' should read P' P" Column 9, line 65, "compressed". should read compressor Column 10, line 49, last figure in formula should read P rather that "P Column 11, line 4, "conduct" should read conduit line 67, "driving" should read dividing Signed and sealed this 5th day of January l97l (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

